The term "thickness" can refer to several different meanings depending on the context in which it is used.
In a general sense, thickness refers to the degree or extent of something - for example, describing how much material makes up an object (e.g., this paper has more thickness than that one). It could also describe the depth or profundity of something, such as a person's thoughts or emotions being described as having great thickness.
In geometry and mathematics, "thickness" refers to the third dimension of an object - specifically, it is the distance between two parallel planes in three-dimensional space. In this context, it can also refer to the thickness of a line or shape.
Finally, in everyday language, "thickness" could be used to describe someone's physical appearance, such as having long hair that adds to their overall thickness.